A robust controller for the system (REF ) can be designed as [1]}
\(&\tau =-e-G e_f-\Delta \tau , ~ \Delta \tau ={\left\lbrace \begin{array}{ll}{\rho }\frac{e_f}{|| e_f||} & ~ \text{if } || e_f|| \ge \varpi \\{\rho }\frac{e_f}{\varpi } & ~ \text{if } || e_f || < \varpi ,\\\end{array}\right.} \\& {\rho }= Y(\xi )^{T} {\Theta } , \) 
